To no kagi no koto (Entrance Opener)

The length of this [tool] is one shaku (approx. 30cm/1f) two sun (one sun is one-tenth of a shaku). The width is one sun wide. The length of the blade is between one and a half to four bu. The length of the handle is one shaku. The iron should be made thin. Make the iron one sun long, and two bu thick.

The purpose of this [tool] is cut the wall of a castle, or it is used to break the fence when you need to enter that castle.

*In essence, this tool is a saw that resembles the basic shape of a kama.

The author indicates that it is used to cut an entrance in the wall of a castle. This is not a mistake. The base of a castle is made of stones, while the walls, at least in some places, are made of wood. Depending on the location, it could be a thin wall, and an easy place to enter. Also, one should not take these instructions literally. That is, the entrance could be a window or a small service door, in which case only the wooden bar locking it should be cut, thus allowing to open it.

Tetsu no koto (Concerning the Iron)

based on the ability to produce an iron blade as displayed in the drawing,

It is better to make the blade six, severn or eight sun